Update, 9:40 p.m.|
Read transportation reporter Mike Lindblom’s full story on the problem, and what officials say it could mean for the Highway 99 tunnel project.
Original post |
The Alaskan Way Viaduct and nearby soil have sunk 1.2 inches this fall alongside stranded tunnel machine Bertha, senior state engineers said Friday afternoon.
The settling shows that the tunnel team is having trouble controlling the soil, crucial to protecting downtown as the Highway 99 tunnel project attempts to move ahead.
A few buildings in historic Pioneer Square have also settled to a lesser extent, the engineers said.
However, neither the viaduct nor the buildings are showing signs of stress such as cracking or buckling, said Matt Preedy, deputy administrator for the tunnel project. He said the viaduct remains safe to drive, as the state collects more data.
Surveys by the state and contractor Seattle Tunnel Partners (STP) confirmed the sinkage 1 1/2 weeks ago, but there hasn’t been further settling this week, said Dave Sowers, a geotechnical engineer at the state Department of Transportation.
This latest worry comes near the one-year anniversary of Bertha’s stall, when the machine overheated and failed to advance Dec 6, 2013.
Contractors are digging a 120-foot deep vertical pit to reach, remove and replace damaged bearing parts at the front of the machine’s 57.3-foot diameter cutter. This phase includes a sophisticated operation to remove water from the soil, which can affect nearby areas.
Dewatering may have caused the soil to sink, said Preedy, but there could also be natural forces, or other construction, at work.
A state survey crew will measure the sides of the viaduct to confirm ground-level and electronic data, and check for any continued slumping.